Plants survive pathogen attack by employing various defense strategies, including strengthening of cell walls, the accumulation of phytoalexins, synthesis of salicylic acid (SA), and induction of pathogenesis-related (PR) genes. A hypersensitive response (HR) is often associated with the defense response and limits pathogen growth to the infected site. After an initial local infection, Systemic Acquired Resistance (SAR) often occurs which coordinately induces expression of a set of PR genes, leading to a long-lasting enhanced resistance against a broad spectrum of pathogens (Durrant and Dong, 2004). In dicots, like Arabidopsis and tobacco, SA and its synthetic analogs, such as 2,6-dichloroisonicotinic acid (INA), benzothiadiazole (BTH), and probenazole are potent inducers of SAR (Ward et al., 1991; Friedrich et al., 1996; Yoshioka et al., 2001). In monocots, SAR can be induced by BTH in wheat (Gorlach et al., 1996) and by Pseudomonas syringae in rice (Smith and Metraux, 1991). BTH can also induce disease resistance in rice (Schweizer et al., 1999; Rohilla et al., 2002; Shimono et al., 2007) and maize (Morris et al., 1998).
The NPR1 (also known as NIM1 and SAI1) gene is a key regulator of SA-mediated SAR in Arabidopsis (Cao et al., 1994; Delaney et al., 1995; Glazebrook et al., 1996; Ryals et al. 1997; Shah et al., 1997). Upon induction by SA, INA, or BTH, NPR1 expression levels are elevated (Cao et al., 1997). NPR1 affects the SAR pathway downstream of the SA signal. Arabidopsis npr1/nim1 mutants are impaired in their ability to induce PR gene expression and mount a SAR response even after treatment with SA or INA. NPR1 encodes a protein with a bipartite nuclear localization sequence and two protein-protein interaction domains: an ankyrin repeat domain and a BTB/POZ domain (Cao et al., 1997). Nuclear localization of NPR1 protein is essential for its function (Kinkema et al., 2000). The ankyrin domain is required for interaction with TGA transcription factors (Zhang et al., 1999; Despres et al., 2000) and the BTB/POZ domain interacts with the repression domain of TGA2 to negate its function (Boyle et al., 2009). During non-induced states, NPR1 protein forms an oligomer and is excluded from the nucleus. Upon SAR induction, monomeric NPR1 emerges through redox changes, accumulates in the nucleus, and activates PR gene expression (Mou et al. 2003). NPR1 also appears to modulate the cross-talk between SA- and JA-dependent pathways; the antagonistic effect of SA on JA signaling requires NPR1, but not nuclear localization of the NPR1 protein (Spoel et al., 2003).
Overexpression of NPR1 in Arabidopsis leads to enhanced disease resistance to both bacterial and oomycete pathogens in a dose-dependent manner (Cao et al., 1998). Similarly, overexpression of Arabidopsis NPR1 or the rice NPR1 ortholog, NH1, in rice results in enhanced resistance to rice bacterial blight pathogen Xanthomonas oryzae pv. oryzae (Xoo) and blast pathogen Magnaporthe grisea (Chern et al. 2001; Yuan et al., 2007), indicating the presence of a similar defense pathway in rice. Although transgenic Arabidopsis plants over-expressing NPR1 acquire enhanced sensitivity to SA and BTH (Freidrich et al., 2001), they display no obvious detrimental morphological changes and do not have elevated PR gene expression until activated by inducers or by infection of pathogens (Cao et al., 1998). However, in rice, overexpression of rice NH1 results in a development- and environment-dependent lesion-mimic phenotype, which can be further enhanced by application of BTH (Chern et al., 2005a). These results suggest that overexpression of NH1 in rice activates the defense response in the absence of inducer treatment or pathogen challenge, an undesirable consequence in terms of practical application. Thus, although rice possesses a pathway similar to the NPR1-mediated one in Arabidopsis, there may be significant differences in their regulation.
There are six NPR1-like genes in Arabidopsis (Liu et al., 2005; Zhang et al., 2006) and five NPR1-like genes in rice (Yuan et al., 2007). Despite extensive investigations done on NPR1, very little is known concerning the NPR1-like genes with regards to their possible involvement in plant defense. Arabidopsis NPR5 and NPR6 have recently been named BOP2 (Blade-On-Petiole2) and BOP1 (Blade-On-Petiole1), respectively. BOP1 and BOP2 regulate Arabidopsis leaf formation. Like NPR1, these proteins function as transcriptional coactivators targeting the AS2 (Asymmetric Leaves2) gene (Jun et al., 2010). Thus, NPR5 (BOP2) and NPR6 (BOP1) are mainly involved in regulating plant development rather than defense. Contradictory results concerning the function of Arabidopsis NPR4 have been reported. Liu et al. (2005) reported that Arabidopsis NPR4 is required for basal resistance to Pseudomonas syringae pv. tomato (Pst) DC3000 and Erysiphe cichoracearum because the npr4-1 mutant is more susceptible to these two pathogens. This group suggested that NPR4 may be also involved in the cross-talk between SA- and JA-dependent signaling pathways since expression of the JA-dependent marker gene PDF1.2 is compromised in npr4-1 leaves following application of methyl-JA. However, Zhang et al. (2006) reported that Arabidopsis NPR3 and NPR4 are negative regulators of PR gene expression and disease resistance. They showed that npr3 mutants have slightly increased basal PR-1 expression and the npr3npr4 double mutant shows even higher PR-1, PR-2, and PR-5 expression. The double mutant plants display enhanced resistance against virulent bacterial (including Pst DC3000) and oomycete pathogens (Zhang et al., 2006). Thus, the roles of NPR4 in disease resistance from these two reports contradict each other. In rice, Yuan et al. (2007) have overexpressed OsNPR1/NH1, OsNPR2, and OsNPR3 in rice and tested for enhanced resistance to Xoo and rice blast. These authors found that only OsNPR1 (but not OsNPR2 or OsNPR3) overexpression conferred enhanced resistance, leaving in doubt whether any rice NPR1 paralogs are involved in defense against pathogens.